Accept hash with indifferent access

Not critical, but for the sake of least surprise ;)

```
Retriever::PageIterator.new(url, { 'maxpages' => 1 }) do |page|  # Works
Retriever::PageIterator.new(url, { maxpages: 1 }) do |page|      # Does not
```

Gonna check out if I can help with it.

Thank you for this great gem, it's a glass of water in crawling hell =D
